Harmonise README + clean LICENSE clause

[mstorath]

Summary

Part of the cross-repo README harmonisation pass (Pass F), plus a LICENSE cleanup.

Two commits on this branch:

1. Harmonise README — full rewrite. The old README (31 lines) was MATLAB-only and made no mention of the dcebe PyPI package despite the package living in this same repo.

   • Add badge block (PyPI / Python / License / CI / MATLAB / File Exchange)
   • Title with TL;DR sentence under hero image
   • Paper citation block at top
   • Python quickstart with estimate_bat
   • MATLAB section preserved
   • Driscoll finite-difference-weights acknowledgement preserved
   • Standardise "See also" to all five sibling repos
   • License footer (Apache-2.0)
   • "Project history" footer mentions Python port and links to PORTED_BY.md

2. Drop non-standard citation clause from LICENSE — the LICENSE file added "if this code is used in any scientific publication, the following paper shall be cited" on top of Apache-2.0. The CITATION.cff and pyproject.toml both declare pure Apache-2.0. Citation request stays in CITATION.cff and README; LICENSE now matches the SPDX expression declared elsewhere in the repo.

Test plan

• Render preview looks correct
• Code snippet runs (from dcebe import estimate_bat)
• License-detector (GitHub sidebar) now identifies the repo as Apache-2.0 cleanly
• Badges resolve once package is on PyPI